It is known to provide paper with sizing or coatings facilitating printing, resisting moisture and for many other purposes, including improvement of the appearance and strength of the paper, reduction of the light permeability (increase of capacity) thereof, etc.
For economical reasons it is desirable to make paper with a minimum sheet thickness and hence a minimum weight. However, the reduction in the sheet thickness or weight results in the need to correlatively reduce the quantity of the coating material which can be applied to a surface. This reduction in coating weight is also made necessary by machine at high speed. It is not uncommon for the paper to pass through the coating machine at speeds of 900 meters per minute, at times up to 1200 meters per minute.
It is desirable to apply the coating in the form of a paste because the mechanical properties thereof (rheology, water retention, fiber coverage) are significant advantages in ensuring uniformity of the coating and the ability to print uniformly and cleanly thereon.
Experience has shown that there is a limit to attempts to reduce the basis weight of paper and hence the sheet thickness. For example, it is not practical to reduce the basis weight of paper below 50 to 52 grams per m.sup.2 because the opacity of the paper is lost and there is a significant reduction of strength. Further when the coating is aplied in as small an amount as 10 g per m.sup.2 on a fiber support with a basis weight of 60 g per m.sup.2, it is found that it is not possible to cover more than 80 to 85% of the surface of the support.
This latter defect is due to the necessity of using baths with proportions of dry material in solution which are relatively low in order to obtain coating of low weights per unit area at high speeds. Because of the need to employ low viscosity baths, it is also necessary to accept a significant penetration of the coating material into the fabrics support either because of migration under the blade or by capillary migration.
In order to eliminate these disadvantages and maintain to a maximum degree the moist film on the surface, it is necessary to increase the concentration of those agents which serve to retain moisture. All this, however, results in an increase in the viscosity of the film and a marked tendency for the coating to gel and thus to form upon drying a rough or wrinkled film which cannot be corrected by calendaring or glossing. The defects in the paper may only be exposed during printing and can result in a large proportion of the printed products being discolored and having surface or other undesirable characteristics. Consequently there is a loss in the product resulting from the compromise of the coating composition between the desire to apply a thin coating to the paper surface and the need to utilize moisture retentive materials to a high degree in such coatings.